This invention relates to a method for the production of solid calcium salts of [S,S]-ethylenediamine-N,N'-disuccinic acid.
Ethylenediamine-N,N'-disuccinic acid (EDDS) and its various alkali metal, alkaline earth metal, ammonium and substituted ammonium salts are well recognized by the detergent industry as useful chelating agents in cleaning formulations. (See U.S. Pat. No. 4,704,233, which is incorporated herein by reference as if fully set forth.) These salts and acids are theorized to chelate metals such as iron, manganese, copper and other multivalent metal ions. The metal ions are constituents of certain organic stains or act to stabilize such stains when present in washing solutions. Besides providing for the chelating function, EDDS and its salts are non-phosphorous compounds and, as a result, are environmentally desirable. Even further, EDDS and its salts exhibit biodegradability. The degree of biodegradability depends upon the optical EDDS isomer involved. Of the three optical isomers, [R,R], [R,S] and [S,S], the [S,S] isomer is most easily biodegradable and is thus preferred.
The [S,S] isomer can be synthesized from L-aspartic acid, generally as the salt, and 1,2-dibromoethane. See for example, the synthesis of [S,S] EDDS from sodium L-aspartate and 1,2-dibromoethane which is reported in Neal and Rose, Stereospecific Ligands and Their Complexes of Ethylenediamine-disuccinic Acid, Inorganic Chemistry, Vol. 7. (1968), pp. 2405-2412. In this synthesis, sodium L-aspartate is reacted with 1,2dibromoethane in a basic aqueous medium. The resultant sodium salt of [S,S] EDDS is soluble in the reaction system and, absent evaporation techniques, is not directly recoverable therefrom as a solid. According to Neal and Rose, the EDDS can be recovered from the solution by slowly acidifying the solution through the addition of concentrated hydrochloric acid to obtain a solution pH of 3.5. The acidification converts the [S,S]-EDDS salt to its acid, which acid crystallizes and precipitates from the solution. Fine crystals are said to precipitate out as the pH moves between pH 7 and 3.5. To purify the EDDS precipitate, which is contaminated with co-precipitates, the solids are recovered and redissolved in a NaOH solution followed by reacidification. The cycle is repeated two times. The final precipitate is washed with water to remove HCl and any traces of L-aspartic acid. While it is implied by Neal and Rose that this purification procedure yields a pure product, the procedure is burdened in terms of procedure time and in terms of HCl utilization due to the multi-cycle purification train.